That is a pretty small panel. Maybe 40 hours from that panel to charge 1 of those batteries.

Here is one approach. Carry your batteries over to Snow Koan camp which is bringing a 30?KW array (big), and also setting one up by the Man, to charge people's batteries, for free, of course. Snow Koan and the Alternative Energy Zone give on playa advice and training in solar. For free!

Bring the batteries, try to buy any other stuff later.

Also good to buy 12v LED lights, make your 12v swamp cooler and all for this year. A plastic tub for the batteries and connections, and a fuse holder with a bunch of fuses is a good idea. Also a volt meter to check the charge on the batteries.

In parallel, your three batteries have a full potential of 384 watt-hours. You really do not want to go much beyond 45% discharge in order to:

1. Recharge them in a reasonable amount of time.2. Prevent more than 50% discharge which decreases the life-span of your batteries by nearly half.

The answer to your question about simultaneous charging and service, the controller should handle both load and charging at the same time. Think of it as the gate keeper to keep juice going where it should and prevent it from going where it should not.

Your panel is very small. Assuming you might use 200 watt-hours per day. it would require your panel to optimally produce for 40 hours per day. The sun doesn't shine for more than 10 useable hours and maybe only SIX of those would be near optimal.

So consider a larger panel capable of producing 40 watts. That would provide you with more than 200 watt-hours per day. That would be pushing your batteries hard to use that much juice every day. But that is a good worst case scenario given the equipment you have.

ETA: That would mean a larger capacity controller. You would need one in the 5-10 amp range. The one you are looking at is 7watts. You need one that could easily handle the 40 watt solar panel. Go with a 60watt (5 amps) controller, and you'll be golden.